Name Servers Are Dispersed
Checks that all of the name servers are on different subnets, for fault tolerance.
A name server in the DNS system is a server that translates domain names that humans understand (example.com) into IP addresses that computers understand (192.0.2.128). Name servers can fail, which is why every domain name should have at least two name servers defined. If one fails, then the other can act as a backup.
To provide even better redundancy, each name server should be set up in geographically different areas. This will protect against the scenario where an entire data center experiences an outage. If your two name servers are in different data centers, then you would be protected from this sort of failure. This test will make sure that none of the name servers are on the same subnet, which likely means they are in the same physical area. If the first three numbers of the IP address are the same, they are on the same subnet.